The present invention relates generally to automotive HVAC systems for controlling the environment of an automobile passenger compartment. More particularly, the invention relates to a battery cooling system for efficiently reusing the heat generated by an electric vehicle battery assembly.
This application is related to co-pending applications all filed on Nov. 12, 1998 and titled Refrigerant Flow Management Center For Automobiles, Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Controller For Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Anti-Fog Controller For Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, Controller For Heating In Reversible Air Conditioning And Heat Pump HVAC System For Electric Vehicles, and Air Handling Controller For Hvac System For Electric Vehicles. Each of these applications is incorporated by reference into the present application.
Automotive heating, ventilation and air conditioning, HVAC, systems have traditionally been single loop designs in which the full volume of refrigerant flows through each component in the system. In an HVAC system, refrigerant in the vapor phase is pressurized by a compressor or pump. The pressurized refrigerant flows through a condenser which is typically configured as a long serpentine coil. As refrigerant flows through the condenser heat energy stored in the refrigerant is radiated to the external environment resulting in the refrigerant transitioning to a liquid phase. The liquefied refrigerant flows from the condenser to an expansion valve located prior to an evaporator. As the liquid flows through the expansion valve it is converted from a high pressure, high temperature liquid to a low pressure, low temperature spray allowing it to absorb heat. The refrigerant flows through the evaporator absorbing heat from the air that is blown through the evaporator fins. When a sufficient amount of heat is absorbed the refrigerant transitions to the vapor phase. Any further heat that is absorbed pushes the vaporized refrigerant into the superheated temperature range where the temperature of the refrigerant increases beyond the saturation temperature. The superheated refrigerant flows from the outlet of the evaporator to the compressor where the cycle repeats. Generally, the refrigerant flowing into the compressor should be in the vapor phase to maximize pumping efficiency. The operation of the refrigerant loop in conventional automotive HVAC systems is controlled by cycling the compressor on and off, and by varying the volume of refrigerant that is permitted to flow through the expansion valve. Increasing the volume of refrigerant that flows through the valve lengthens the distance traversed by the liquid before it changes to the vapor phase, allowing the heat exchanger to operate at maximum efficiency.
Advances in automotive HVAC systems have led to zone temperature control systems wherein different zones of an automobile are independently controlled. Zone control systems generally include an evaporator and expansion valve for each zone. The refrigerant flows through a compressor and condenser, then is split by a system of valves before flowing to the expansion valve and evaporator of each zone. The refrigerant flowing out of the evaporator of each zone is then recombined before returning to the compressor. Heat that is transferred to the refrigerant from each of the evaporators is shed to the external environment as the refrigerant flows through the condenser. A complex series of valves and plumbing is generally required to maintain a balanced HVAC system that provides individualized cooling control for each of the zones.
With the advent of electric vehicles reversible heat pump systems have been introduced into automobiles. In a reversible heat pump system the HVAC system can either heat or cool a compartment depending on the direction of the refrigerant flow. In the air conditioning mode refrigerant flows from the compressor through an outside coil (condenser) and into an expansion valve and inside coil (evaporator) before returning to the compressor. Heat energy is extracted from air that is blown through the inside coil (evaporator) into the passenger compartment thus providing cooled air. In the heating mode a four way valve reverses the flow of refrigerant through the coils, thereby reversing the function of the coils. Refrigerant flows from the compressor through the inside coil (condenser) then into an expansion valve and the outside coil (evaporator) before returning to the compressor. Heat energy in the liquefied refrigerant flowing through the inside coil is absorbed by air that is blown through the coil into the passenger compartment thus providing heated air.
Generally, the amount of heat that can be transferred from the evaporator in an HVAC system to the condenser is limited by the system components as well as the temperature of the outside air. Therefore, under some temperature conditions a heat pump system will not be able to provide sufficient heat to warm the passenger compartment to the desired temperature. Under these conditions another heater such as an electric heater is required to provide the desired heating. A drawback of electric heaters is that they are less energy efficient than a heat pump. To supply heat to the passenger compartment electric heaters such as PTC heaters convert energy drawn from the vehicle electrical system. Therefore, energy directed towards heating reduces the energy available for propulsion of the vehicle, thereby reducing the overall efficiency. A heat pump on the other hand, uses a small amount of energy to drive a compressor which extracts heat energy from the external environment and transfers it into the passenger compartment. A heat pump uses about 40% of the energy that a PTC heater would use to heat the passenger compartment to a desired temperature.
Electric vehicles have an additional constraint in that the vehicle batteries must be cooled. In an electric vehicle, propulsion is provided by an assembly of batteries that store energy. The energy stored in the batteries supplies electric motors which drive the vehicle wheels. When energy is stored to or discharged from the battery assembly, heat is generated within the battery assembly. Conventional electric vehicle designs cool the battery assembly with a heat exchanger circuit and then exhaust the heat that was absorbed from the batteries to the outside environment.
Heating the passenger compartment of an electric vehicle causes a significant energy load on the vehicle battery assembly, especially so when electric heaters are used. Cooling the battery assembly of an electric vehicle requires energy from the battery assembly further depleting the energy available for propulsion of the vehicle. Energy diverted from the battery assembly towards heating the passenger compartment reduces the operating range of the vehicle, increases operating costs, and reduces the lifetime of the vehicle batteries by subjecting them to deeper discharge cycles.
One object of the present invention is to provide a system for increasing the operating range of an automotive heat pump system.
Another object of the present invention is to provide an energy efficient method for cooling the battery pack of an electric vehicle.
It is an additional object of the present invention to provide a system for improving the energy efficiency of an electric automobile.
A further object of the present invention is to provide a system for efficiently distributing the heat energy of an electric automobile.
Accordingly, the invention provides a battery cooling system to cool an electric vehicle battery pack and extend the operating range of an automotive heat pump system. Waste heat from the battery pack is transferred to a secondary coolant system thereby cooling the battery pack. A secondary heat exchanger is coupled between the secondary cooling system and a reversible HVAC system. The secondary heat exchanger transfers the secondary coolant system heat energy to the reversible HVAC system. The heat energy transferred to the reversible HVAC system supplements the existing stored energy thereby extending the heating mode operating range of the HVAC system.
The above described device is only an example. Devices in accordance with the present invention may be implemented in a variety of ways.